—Disease patterns, observing how a problem spreads. Gun ownership — a precursor to gun violence — can spread "much like an infectious disease circulates," said Daniel Webster, a health policy expert and co-director of the Johns Hopkins Center for Gun Policy and Research in Baltimore.

And then he explains the metaphor...

"There's sort of a contagion phenomenon" after a shooting, where people feel they need to have a gun for protection or retaliation, he said.

Are you going to say that gun sales don't jump up after mass shootings?